In 2006, California passed its groundbreaking climate legislation AB 32, which put in place a target for greenhouse gas reductions and set in motion a cascade of regulations, subsidies, and performance standards that has continued unabated ever since.

Three years after that, in 2009, a nonprofit advocacy organization called Next 10 teamed up with the research firm Beacon Economics to track the state’s progress in a detailed annual report called the California Green Innovation Index.

The ninth edition of the CGII has just been released, and it offers a good opportunity to reflect on how California has done so far and, more importantly, to grapple with the big challenge that lies just ahead.

To put it as simply as possible: California’s experience shows that decarbonizing the electricity sector is both possible and profitable, but to reach its ambitious carbon targets, the state will now have to decarbonize transportation — which brings a whole new and daunting set of difficulties.

As has so often been the case, California is a few steps ahead of the rest of the country in this, offering a preview of things to come. The state’s biggest decarbonization problem — cars — will soon become the nation’s.